1. Field of the Invention
The present invention relates to a vibration isolation apparatus provided for a vehicle, in particular, for an engine mount and other such parts of the vehicle, to absorb vibration from a vibration producing portion.
2. Description of the Related Art
A vibration isolation apparatus is disposed in the vehicle engine as an engine mount between an engine and a vehicle body. The vibration isolation apparatus impedes the transmission of vibration from the engine to the vehicle body. The engine vibrates in various modes of vibration. For example, a shake vibration is generated when the vehicle is traveling at speeds of about 70 km/h, and an idle vibration is generated when the engine idles or the vehicle is traveling at speeds of about 5 km/h. In general, the frequency range of the idle vibration is 20 to 40 Hz while that of the shake vibration is less than 15 Hz. Therefore, the shake vibration and the idle vibration have different frequencies.
A fluid filled-type vibration isolation apparatus, is disclosed in Japanese Patent Application Laid-Open No. 62- 101935 as a vibration isolation apparatus which can effectively absorb vibrations of such a wide range of frequencies.
This vibration isolation apparatus is equipped with a main fluid chamber and an auxiliary fluid chamber which are filled with fluid in an outer cylinder. The main fluid chamber and the auxiliary fluid chamber communicate with each other through a through-path having a small diameter and a through-path having a large diameter. The main fluid chamber and the auxiliary fluid chamber always communicate with each other through the through-path having a small diameter. In addition, the main fluid chamber and the auxiliary chamber communicate through the through-path having a large diameter via an opening and closing valve which is provided on an outer periphery of the outer cylinder.
When the engine vibrates at a low frequency, the opening and closing valve is closed, and the fluid flows back and forth only through the through-path having a small diameter. Therefore, the vibration can be absorbed by flow resistance which is generated when the fluid flows through the through-path having a small diameter. On the other hand, since the through-path having a small diameter is blocked when the engine vibrates at a high frequency, the opening and closing valve is opened so that the fluid flows through the through-path having a large diameter. As a result, resonance of the fluid is caused in the through-path having a large diameter to decrease a dynamic spring constant of the vibration isolation apparatus and absorb the vibration.
However, since the through-paths and the opening and closing valve, which opens and closes the through-paths, are provided separately for the vibration isolation apparatus, they must be assembled onto the outer cylinder after they are assembled separately. Accordingly, assembly becomes complicated, and it is difficult to make the whole vibration isolation apparatus compact.